Wax based inks, sometimes known as phase change inks or solid inks, are used to form digital images on paper using heated piezoelectric or acoustic inkjet heads. The ejection of an ink drop by the print heads is electronically controlled. In embodiments, the hot drop partially cools upon hitting an intermediate surface, often an aluminum drum. The complete image is assembled on the intermediate surface and then transferred to paper and fixed thereon with a combination of pressure and heat resulting in a solid ink, or waxed-based ink print. Alternatively, the ink can be ejected directly onto the final print substrate such as paper to form the printed image.
However, such solid inks have suffered from a lack of robustness, manifested as poor resistance to scratch, rub, fold, and other outside influences on the final print. Overcoatings and undercoats have long been proposed as a solution to this problem. Overcoats act as a protective layer for a non-robust ink film, while undercoats serve as an adhesion interface between the ink image and the underlying print substrate. In either case, if applied to the entire page, the coatings add significant cost to the per page cost of printing. Digital overcoats have been attempted, but because the viscosity requirements of jetting are imposed on these materials, they are often not significantly more durable than the ink layer itself, thus they merely act as a sacrificial layer, which does not truly improve image robustness. Curable coatings may solve the robustness problem, but add cost and complexity to the printer, as a curing station must be incorporated.
Known methods of protecting ink jet printed images include applying a coating, such as an overcoating over the printed image and substrate or an undercoating between the image and the substrate. The overprint coating, often referred to as an overprint varnish or composition, is typically a liquid film coating that may be dried and/or cured. Curing may be accomplished through drying or heating or by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overprint coatings fail to adequately protect the printed images and do not possess the requisite properties for controlled application, such as, for example, by an ink jet printer.
Other types of coating formulations for ink-based images are also known. For example, UV curable ink-jet inks have been used in an attempt to overcome the failure of ink jet-generated images to withstand heat and sunlight. Typically, such UV curable ink-jet inks include polymerizable monomers, oligomers, or a mixture thereof along with a dye or pigment for color. However, these ink-jet inks often contain relatively large amounts of toxic solvent or water, as described in U.S. Pat. Nos. 4,978,969 and 5,623,001, respectively, or other toxic components, such as the varnish described in U.S. Pat. No. 5,270,368, or require specific, impractical conditions, such as, the varnish described in U.S. Pat. No. 4,303,924.
Typically, known coating formulations are applied using a liquid film coating device, and thus are often applied over the entire surface of the substrate and the image, i.e., flood coating. Applying a composition to part of an image, i.e., spot coating, is possible, but it requires preparing a plate or cylinder prior to application of the overprint composition. Thus, applying known coating formulations can be inefficient, difficult, and time-consuming and is not desirable for integration with variable-data digital printing.
Accordingly, there is a need for a strongly adhesive material having very low melt viscosity that can be jetted at typical SIJ operating temperatures as an adherent undercoat between the substrate and the printed image. More specifically, a need exists for an underprint coating that has a low viscosity (to enable ink jetting), yet is stable at the generally high temperature required for ink jetting and achieves the desired properties, including, but not limited to, the ability to wet over the substrate and adhere to both the substrate and the applied ink image.